Watermarking of audio signals intends to manipulate the audio signal in a way that the changes in the audio content cannot be recognised by the human auditory system. Most audio watermarking technologies add to the original audio signal a spread spectrum signal covering the whole frequency spectrum of the audio signal, or insert into the original audio signal one or more carriers which are modulated with a spread spectrum signal. There are many possibilities of watermarking to a more or less audible degree, and in a more or less robust way. The currently most prominent technology uses a psycho-acoustically shaped spread spectrum, see for instance WO-A-97/33391 and U.S. Pat. No. 6,061,793. This technology offers a good compromise between audibility and robustness, although its robustness is suboptimal.
Many audio watermarking technologies like spread-spectrum, or phase shaping as disclosed in US-A-2005/0033579 and in EP 05090261.8, use correlation with a reference sequence for detection in the decoder. The robustness of the watermark is improved by increasing the length of the reference sequence.